Photolithography is a patterning process in which a patterned design is transferred from a photomask to a photoresist material as part of a substrate patterning process. The photomask is a lithographic template that includes the patterned design to be transferred to the photoresist material. Conventionally, the photomask includes an opaque surface with a plurality of open or radiation transparent portions that form the patterned design to be transferred. Radiation incident on the photomask passes through the open portions of the photomask to expose portions of the photoresist material in a pattern corresponding to the open portions of the photomask. Radiation incident on the opaque surface of the photomask does not pass through the photomask and the photoresist material remains unexposed in a pattern corresponding to the opaque surface of the photomask.
If positive photoresist material is used, the exposed portions of the photoresist material are removed by a photoresist developer, while the unexposed portions of the photoresist material remain on the substrate. If negative photoresist material is used, the exposed portions of the photoresist material remain on the substrate, while the unexposed portions of the photoresist material are removed by a photoresist developer. After development, one or more physically exposed portions of the substrate may be patterned by an etching process, while physically unexposed portions of the substrate remain covered by the photoresist material. After etching, remaining portions of the photoresist material are removed. In operation, the photomask may be used to replicate a patterned design on a plurality of substrates as part of the substrate patterning process. Depending on the type of photoresist used (positive or negative) and the type of photomask used (i.e., bright field or dark field), the photomask may be used to replicate a positive image of the photomask patterned design or a negative image of the photomask patterned design on one or more substrates. Photomasks are commonly used in the fabrication of semiconductor devices.